Molded case industrial circuit breakers provided with a movable contact arm capable of pivoting to an open position called the "blow open position" independent of the circuit breaker operating mechanism are capable of limiting the current through the contacts under short circuit fault conditions. The rapid separation of the contacts in the early stages of the current waveform effectively limits the total amount of current between the contacts when the arc forms upon such separation. Earlier contact arm and contact spring arrangements for such current limiting are disclosed in U.S. patent application Ser. No. 479,617 filed Mar. 28, 1983 entitled "Electric Circuit Breakers Having Fast Short Circuit Response', and U.S. patent application Ser. No. 479,616 filed Mar. 28, 1983 and entitled "Variable Torque Contact Arm For Electric Circuit Breakers". Both of these patent applications are incorporated herein for purposes of reference. The main advantage achieved by current limiting is the reduction of the effective thermal energy dissipated within the breaker measured as the product of the square of the arc current multiplied by the time that the arc continues to exist. By interrupting the current through the breaker early in the current waveform, however, the voltage across the breaker components is at a high value. The arc energy, as measured by the product of the voltage and the current multiplied by the time that the arc persists, is also at a relatively high value. The higher the arcing energy within the breaker, the larger the arc chute requirement in order to absorb the arc energy without damaging the breaker components.
The main advantage achieved by the "current limiting" provided by the independently pivoting contact arm upon short circuit fault conditions is the wide range of current ratings that can be safely handled by a single breaker geometry. Heretofore, it has been customary to increase the size of the breaker components in proportion to the current rating which usually resulted in a larger overall breaker geometry. As described within the aforementioned U.S. Patent Applications, some components within the standard size breaker geometry must be increased in order to deter overheating during normal operating conditions. One such component is the flexible braid which is attached to the movable contact arm on the line side of the circuit breaker. The large braid could present a space problem since the contact arm upon opening independent of the operating mechanism repositions the braid such that it comes in near contact with the breaker components which are operatively positioned by means of the operating mechanism.
The arcing energy that is generated during a circuit interruption must not exceed the energy that a breaker can withstand without damage. As described earlier, one breaker design can have different voltage and current ratings. Lower system voltages, for example, generally have higher current interrupting ratings.
The purpose of this invention is to provide a contact arm and contact spring configuration that allows more effective current limiting on low voltage systems where the available current is higher, and less current limiting on higher voltage systems where the available current is lower. A further purpose of the invention is to provide means for allowing the braid to move within the breaker housing without interfering with any other breaker components when the operating mechanism is bypassed and the contact arm operates under short circuit conditions.